A hydraulic system is a system configured to control, by using hydraulic control valves (e.g., a pressure control valve, a solenoid operated switching valve, and a flow rate control valve), at least one of the pressure, direction, and flow rate of pressure oil discharged from a hydraulic pump to a hydraulic actuator (e.g., a single rod hydraulic cylinder or a hydraulic motor). Such hydraulic systems are widely used in the fields of, for example, construction machinery, industrial vehicles, industrial machinery, and ships and vessels. There are cases where a hydraulic controller that forms a part of such a hydraulic system includes an accumulator as an auxiliary power source for the purpose of reducing the size of the hydraulic pump as well as in consideration of an emergency situation where the hydraulic pump breaks down or a power failure occurs. The accumulator is a hydraulic device configured to store hydraulic energy. The accumulator which stores such energy may be of a gas loaded type, spring loaded type, or weight loaded type. The accumulator may store hydraulic pressure in any of the methods as described below.
A first pressure storing method is a method of storing pressure by using a pump dedicated for the storing of pressure, which pump is installed separately from the hydraulic pump which drives the hydraulic actuator. For example, Patent Literature 1 discloses in paragraph 0006 that in the case of a conventional hydraulic circuit, it is necessary to install an electric motor dedicated for driving a pressure storage pump used for storing pressure in an actuator’.
A second pressure storing method is a method of storing pressure when the hydraulic pump remains idle. This method is adopted in a case, for example, where equipment including the hydraulic pump often performs a pressure holding operation in which the flow rate of an inflow to a main circuit may be small, or where a pressure storing mode is performed between cycle operations in which the hydraulic actuator is operated intermittently. For example, Patent Literature 2 discloses in paragraph 0039 that ‘pressure oil supplied from a pressure oil supply device during an idle time of a single rod hydraulic cylinder unit is stored in a pressure oil chamber of an accumulator’.
A third pressure storing method is a method of storing pressure by utilizing surplus oil that is produced when the hydraulic actuator is driven by pressure oil discharged from the hydraulic pump. For example, Patent Literature 3 discloses in paragraph 0013 that ‘accumulator means stores the pressure of pressure oil, the pressure of which has been increased by pressure increasing means using surplus oil fed from hydraulic control means, and the pressure increasing means is, for example, a single rod hydraulic cylinder configured to increase the pressure of surplus oil by using the pressure of the surplus oil, or a high pressure pump configured to increase the pressure of pressure oil by using a driving force of a hydraulic motor, the driving force of which is generated by the pressure of surplus oil (here, in the case where the pressure increasing means is the single rod hydraulic cylinder, the accumulator means stores the surplus oil)’.